Passing an enum to a field. How can it be?

[Daniele Barell]

Hi ranchers!
There's a code I came across and I cannot fully understand:

enum Colors{
	RED(0xff_00_00),GREEN(0x00_ff_00),BLUE(0x00_00_ff);
	int hex;
	Colors(int hex){ this.hex = hex;}
}
public class TestColors{
	static Colors c;
	public static void main(String[] args){
		System.out.println(c.RED.hex + " " + c.BLUE.hex);
	}
}

It compiles regularly and print two numbers.

What puzzles me is the following line:

static Colors c;

What is c? Shouldn't be defaulted  as null? Or Should it be assigned to a Colors enum value like c = Colors.RED? How can it represet an entire enum?

[Junilu Lacar]

c is a reference to an enum, which is defined as any of the values listed: RED, GREEN, BLUE.

[Junilu Lacar]

Since enums are objects, yes, they will be initialized to null if they are class/instance members. Edit (clarification): A reference to an enum, like c, will be initialized to null if it is a class or instance member.

[Junilu Lacar]

Your confusion may stem from the misleading name given to the enum type. Just like classes, an enum is usually given a singular name, like Color in this case, not plural.

[Paul Clapham]

Yes, the variable c is initialized to null and never changed to any other value. However the expression c.RED doesn't actually use the variable c. It's actually equivalent to Colors.RED; I'm not sure why the designers of Java made this legal but they did.

[Daniele Barell]

Hi (again) Junilu.
Thanks for all the answers you provide!

Junilu Lacar wrote:Since enums are objects, yes, they will be initialized to null if they are class/instance members. Edit (clarification): A reference to an enum, like c, will be initialized to null if it is a class or instance member.

So how can be possible to work the print output?

System.out.println(c.RED.hex + " " + c.BLUE.hex);

[Daniele Barell]

Hi Paul

Paul Clapham wrote:Yes, the variable c is initialized to null and never changed to any other value. However the expression c.RED doesn't actually use the variable c. It's actually equivalent to Colors.RED; I'm not sure why the designers of Java made this legal but they did.

That's odd, isn't it?

[Paul Clapham]

Yes, it is. You didn't mention it but that makes me suspect your code came from something which is aimed at people taking a certification exam. I wouldn't expect to find that "feature" in real code anywhere.

[Junilu Lacar]

Java compiles an enum into something very similar to this regular class definition:

public class Color {
  
    public static final Color RED = new Color(..);
    public static final Color GREEN = new Color(..);
    public static final Color BLUE = new Color(..);

    ...

    private Color(...) {
    }
}

Since static members of a class can also be accessed via a variable reference, I guess it makes sense to allow access to an enum value via a variable reference. It's confusing though, as you have seen, so you should avoid doing that.

[Junilu Lacar]

This works, too:

public class Main {
  static Color c;
  public static void main(String[] args) {
    System.out.println(c.RED.val);
  } 
}

class Color {
  public static final Color RED = new Color(2);
  int val;
  private Color(int val) {
    this.val = val;
  }
}

You can see for yourself here: https://repl.it/Mb9D/1

[Daniele Barell]

Paul Clapham wrote:Yes, it is. You didn't mention it but that makes me suspect your code came from something which is aimed at people taking a certification exam. I wouldn't expect to find that "feature" in real code anywhere.

What an eye! :)
Actually I took and "adapted" a question from a book on OCAJP I'm studied.

[Carey Brown]

Paul Clapham wrote:Yes, the variable c is initialized to null and never changed to any other value. However the expression c.RED doesn't actually use the variable c. It's actually equivalent to Colors.RED; I'm not sure why the designers of Java made this legal but they did.

Color.RED is the preferable way to access static members. Then you can do away with 'c' entirely.

[Daniele Barell]

Junilu Lacar wrote:
You can see for yourself here: https://repl.it/Mb9D/1

cute!

[Paul Clapham]

Junilu Lacar wrote:Since static members of a class can also be accessed via a variable reference, I guess it makes sense to allow access to an enum value via a variable reference.

Yes, I'd seen the former usage before (discussed here on the Ranch) but not the latter. But yeah, I guess they are the same thing.